Commonly optical Raman fiber amplifiers are known where light with a pump frequency in an optical fiber amplifies light which corresponds to a Stokes line of the Raman process. The frequency difference between the pump light and the light to be amplified may be typically in the range of 40 nm to 80 nm such as 60 nm.
The Raman process is enhanced e.g. by germanium as an impurity and hence e.g. germanium doped silica fibers are often used for Raman amplifiers. Currently existing Raman amplifiers can reach high powers, but not narrow amplified linewidths at the same time.
Several problems in Raman fiber amplification can be identified. For example, the stimulated Brillouin (SBS) light scattering or Four-Wave mixing (FWM) lead to a linewidth enhancement. This may be undesired for various reasons. A broader linewidth is not favorable for spectroscopic resolution. Further this linewidth broadening usually introduces losses since such light out of a resonant mode or out of a desired wavelength range will be lost.
Raman amplification relies on fiber nonlinearity. This nonlinearity can be described by a tensor of the third level, often referred to as the X(3)-tensor (see e.g. G. P. Agrawal, “Nonlinear Fiber Optics”, 4th ed., Academic Press, USA). Unfortunately, this nonlinearity also includes other unwanted effects such as four-wave mixing (FWM) and stimulated Brillouin scattering (SBS). The strength of the nonlinearity is commonly characterized by the scalar quantity
                                          n            2                    =                                    3                              8                ⁢                                                                  ⁢                n                                      ⁢                          Re              ⁡                              (                                  χ                  xxxx                                      (                    3                    )                                                  )                                                    ,                            (        1.1        )            where n is the linear part of the refractive index of the fiber, Re( ) denotes the real part, and xxxx stands for one specific tensor element.
FWM leads to the mixing of different spectral components and induces line broadening. Also, light power at a desired frequency can mix with optical noise and hence signal energy is lost.
Specifically, SBS is a serious detriment in fiber lasers: High laser light intensities create acoustic phonons in the fiber that induce a refractive index grating. The light is backscattered from this grating, an effect which both limits the maximum transmitted power in the fiber and may destroy previous amplifier stages or a seed laser. The onset of SBS happens suddenly when exceeding a certain power level. This power level depends on a number of system parameters such as fiber length, bandwidth of the light, and fiber nonlinearity. In particular, the fiber glass composition, possible glass dopants, and the fiber index profile have a strong influence. In general, SBS will be most severe (start at the lowest power threshold) when using narrowband signals in long, uniform optical fibers. These circumstances have limited the use of Raman amplifiers to either wideband and/or low power applications in the past.
Stimulated Raman amplification (SRS) can be mathematically described as
                                                        ⅆ                              I                s                                                    ⅆ              z                                =                                                    g                R                            ⁢                              I                s                            ⁢                              I                p                                      -                                          α                s                            ⁢                              I                s                                                    ,                            (        0.1        )            where z is distance along the fiber, Is denotes the signal intensity, Ip the pump intensity, gR the Raman gain coefficient, and αs denotes linear light attenuation. A similar equation holds for SBS:
                                                        ⅆ                              I                s                                                    ⅆ              z                                =                                                    g                B                            ⁢                              I                s                            ⁢                              I                p                                      -                                          α                s                            ⁢                              I                s                                                    ,                            (        0.2        )            where gB is the Brillouin gain coefficient and the first minus sign indicates power loss.